One of the major uses for the typical multiplier tube is in the field of scintillation counting. When a particle of ionizing radiation leaves energy in a suitable material such as a phosphor, a weak pulse of light is emitted which is detected by the multiplier tube. The photocathode of the tube is responsive to the incident light to develop electrons in proportion to the intensity of the incident light. The electrons emitted by the photocathode are directed so they strike an electrode known as the first dynode. The energy of incident electrons is given to electrons in the dynode and, on an average, several electrons are emitted from the surface of the dynode for each incident electron. This process is known as secondary emission and results in a net multiplication or gain in the number of electrons per radiation event. The emitted electrons are then directed to a second dynode where their number is again multiplied by the secondary emission process. Further multiplication takes place at succeeding dynodes until the electrons are collected at the final dynode or the tube anode. With 10 or more dynodes the multiplication factor of the tube or gain can be very great, up to 10.sup.8.
One particular problem associated with multiplier scintillation detectors is that as the rate at which radiation events to which the tube is to be responsive varies, shifts in the value of the multiplication factor or gain of the tube occur. Thus for one rate of occurrence of radiation events the gain of the tube will reach an equilibrium gain of one value, while for another rate the tube will reach another gain. These gain shifts are characterized not only by different tube outputs but also by a long time constant for the tube to reach the equilibrium gain. This necessitates continual monitoring and recalibration of the tube to utilize tube output amplitude for analytical purposes.
It is therefore an object of this invention to provide a means of operating a multiplier tube with reduced gain shifts due to varying event count rates.
Another object of this invention is to reduce the time constant associated with the multiplier tube reaching an equilibrium gain in response to varying event count rates.